The INSIDER Summary:

Maisie Williams shared three photos of her and "Game of Thrones" costar Sophie Turner.

The photos were shared on Instagram with a simple caption: "Happy 7th August."

Fans will know that's the anniversary of the day both actresses were cast on the popular HBO show.

Since then, the two have become best friends.

The two even share a tattoo of the date together.

"Game of Thrones" actresses Maisie Williams and Sophie Turner don't just play sisters on TV. They also consider themselves best friends. The two look out for each other on the red carpet and Turner recently told INSIDER she considers Williams her "soul mate."

Williams took to Instagram Monday afternoon to share three photos of herself and Turner, who are known to fans as "Mophie."

While the photos themselves are adorable — they're two black and white pictures along with a face swap — it's the caption that has a lot of emotional weight in it.

The simple caption reads, "Happy 7th August."

For outsiders, Williams could just be saying "Happy Monday," but for Mophie fans, it holds a deeper meaning. August 7 marks the anniversary of when Sophie Turner and Maisie Williams were first cast on "Game of Thrones" eight years ago in 2009.

Last year, the two got matching tattoos on the insides of their arms with the date as a symbol of their time on the show in case either of them were killed off unexpectedly.

Williams showed off her tattoo at the BAFTA Tea Party in London in September 2016.
Richard Shotwell/Invision/AP

Turner and Williams told Entertainment Weekly it was a bit tough to be on screen together when they're used to casually hanging out. It turns out bringing your A-game in front of your bestie can be a bit intimidating.

"It took a few hours for us to get serious and actually do it," said Williams. "Later it was fine on set and we could concentrate, but it was so hard to get into character with Sophie in front of me."

We're looking forward to more moments with the Stark sisters together on screen and off.